A photographic process obtaining direct positive images without requiring an additional reversal processing step or a negative film is well-known
A conventional process which is used for forming positive images using a direct positive silver halide photographic material is mainly classified into the following two types, apart from specific ones, considering their practical utility.
In one type of photographic process, a direct positive image is obtained by using a previously fogged silver halide emulsion and, after development, destroying fogged nuclei (latent images) at exposed portions by utilizing solarization or the Herschell effect.
In another type of photographic process, a direct positive image is obtained by using an unfogged internal latent image-type silver halide emulsion and, after image-wise exposure, applying surface development after or during applying fogging treatment.
Also, the aforesaid internal latent image-type silver halide photographic emulsion include a silver halide photographic emulsion of a type having sensitivity speck mainly in the inside of silver halide grain and forming the latent image mainly in the inside of the grain by light-exposure.
The latter type process is generally high in sensitivity as compared to the former type process and is suitable for uses requiring high sensitivity. The present invention relates to the latter type.
Various techniques are known in the technical field. For example, these techniques are described in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276, 3,796,577, British Pat. Nos. 1,151,363, 1,150,553, 1,011,062, Research Disclosure, RD No. 15162 (November, 1976), ibid., RD No. 17643 (December, 1978), etc.
By using these known processes, relatively high sensitivity direct positive photographic light-sensitive materials can be obtained.
Also, details of the mechanics of forming direct positive images are described in T. H. James, The Theory of the Photographic Process, 4th edition, Chapter 7, pp. 182-193, and U.S. Pat. No. 3,761,276.
That is, it is believed that photographic images (direct positive images) are formed at unexposed portions of a photographic light-sensitive material by selectively forming fogged nuclei on the surface only of the silver halide grains at the unexposed portions due to the surface desensitizing action caused by so-called internal latent images formed in the inside of the silver halide by a first imagewise exposure an then applying an ordinary so-called surface development process to the photographic light-sensitive material.
As a means for selectively forming fogged nuclei as described above, a so-called "light fogging method" applying a second light exposure onto the whole surface of a light-sensitive layer (as described, for example, in British Pat. No. 1,151,363) and a so-called "chemical fogging method" using a nucleating agent are known. The latter method is described, for example, in Research Disclosure. Vol. 151, RD No. 15162, pp. 76-78 (November, 1976).
Direct positive images are formed by applying a surface color development process to an internal latent image-type silver halide light-sensitive material after or during the application of a fogging treatment, using light or a nucleating agent, to the light-sensitive material and thereafter subjecting the light-sensitive material to a bleaching and fixing process (or blix process). After the fixing process or blix process, an ordinary wash process and/or a stabilization process is applied.
In such direct positive image formation using the light fogging method or chemical fogging method, the processing time is long since development speed is slow as compared with processing ordinary negative-type photographic light-sensitive materials. If the pH of the developer is lowered, the processing time becomes longer and hence it is not preferred to reduce the pH of the developer in the direct positive image formation.
Accordingly, it is difficult to obtain a direct positive image having high maximum image density and low minimum image density by using a developer having low pH and, hence, a process of shortening the processing time by increasing the pH and/or the temperature of developer has been employed.
Furthermore, when a direct positive photographic light-sensitive material is developable with a low-pH developer, photographic stability, that is, storage stability of unexposed light-sensitive material before photographic processing, is usually reduced and this is assumed to be caused by the fact that the emulsion components of such a light-sensitive material has slight activity in the pH range (usually from 5 to 7) during storage of the unexposed light-sensitive material. Accordingly, a photographic light-sensitive material which has a nucleating action in only high-pH development processing is necessary for obtaining good storage stability of the unexposed light-sensitive material. However, the above described processing property in a low-pH developer is inconsistent with good storage stability in the unexposed state and this problem is particularly acute in the chemical fogging method.
However, a developer having high pH generally causes the problem that the minimum image density of a direct positive image formed is increased. Also, under high pH, deterioration of a developing agent due to air oxidation is liable to occur, which results in greatly reducing development activity.
For solving these problems, compounds providing nucleating action at a pH even less than pH 12 are proposed in Japanese Patent Application (OPI) No. 69613/77 (corresponding to U.S. Pat. 4,115,122) (the term "OPI" as used herein referred to a "published unexamined Japanese patent application") and U.S. Pat. Nos. 3,615,615 and 3,850,638. However, even when these compounds are used, minimum image density is increased and, furthermore, these nucleating agents have a disadvantage that during the storage of the light-sensitive materials before processing, the nucleating agents react with silver halide or the nucleating agent itself is decomposed to reduce, finally, the maximum image density of the light-sensitive material after processing.
Other means for increasing development speed in direct positive image formation are as follows.
It is described in U.S. Pat. No. 3,227,552 that development speed for an intermediate density is increased by using hydroquinone derivatives.
Also, Japanese Patent Application (OPI) No. 70843/85 describes that the maximum image density is increased by adding a mercapto compound having a carboxylic acid group or a sulfonic acid group. However, the effect due to the addition of the aforesaid compound is small and, further, the pH of the developer in this case is 12.0, which is not low enough to be considered a reduction of the pH of the developer.
Japanese Patent Application (OPI) No. 134848/80 describes a light-sensitive material processed by a processing liquid (pH 12.0) containing a tetraazaindene compound in the presence of a nucleating agent to reduce the minimum image density, thereby the formation of re-reversed negative images is prevented.
However, a stable technique for obtaining a direct positive image having a high maximum image density by processing for a short period of time using a low-pH developer using any of the techniques described above or using a combination of these techniques along with a light-sensitive material for use in such a technique which has good storage stability in the unexposed state, has not yet been found.
On the other hand, various methods have been proposed in the field of the art for increasing development speed and coloring speed of the color developer. Methods which depend upon a color developing agent, which must be permeated into coupler-dispersed oil drops for finally forming dyes by causing coupling with couplers, combined with various kinds of additives for accelerating coloring by hastening the permeation of the color developing agent are known. One compound which is known to have particularly great color formation accelerating effect is benzyl alcohol. Benzyl alcohol has been used for processing various color photographic materials and has at present been used as a necessary component in the field of photographic processing.
Benzyl alcohol may dissolve in water to some extent, but is poor in water solubility. For improving solubility, diethylene glycol, triethylene glycol or alkanolamine have been widely used.
However, these compounds and benzyl alcohol itself have a high BOD value and COD value resulting in a substantial pollution load when processing the developer as waste liquid. Hence, in spite of the above-described color properties and solubility advantages, it is desired from the point of waste liquid processing to reduce or omit benzyl alcohol in the developer.
Furthermore, the solubility of benzyl alcohol is insufficient even using one of the aforesaid solvents, such as diethylene glycol, which results in increased work and time required for preparing the developer.
Also, if benzyl alcohol is carried over to a bleach bath or a blix bath from a development bath containing benzyl alcohol by light-sensitive materials and accumulates in the bleach bath or blix bath, the benzyl alcohol reacts in the bleach bath or blix bath to form a leuco compound according to the kind of a cyan dye formed by development, which results in reducing color density.
Furthermore, the accumulation of benzyl alcohol causes insufficient washing out of developer components. In particular, the accumulation of benzyl alcohol makes it difficult to wash out the color developing agents from color photographic material in the wash step and, hence, these remaining components cause deterioration of the storage stability of the color images formed.
From these reasons, reducing or removing benzyl alcohol in a color developer is a significant and desirable objective.
At present, benzyl alcohol has the problems described above and, on the other hand, shortening processing time is necessary to meet the demand for quick delivery of the finished product.
However, these requirements are not simultaneously satisfied by conventional techniques and if benzyl alcohol is removed from color developer and the development time is shortened, color density is, as a matter of course, greatly reduced.